In the field of thermal machines (for example gas turbines) and boilers, it is of very major importance to know the actual operating temperatures (typically 700-1000° C.) at various points in the apparatus. However, it is frequently impossible to retrospectively additionally fit active (wired) sensors in an apparatus which is already in use, as a result of which passive sensors, which indicate the temperature by a (permanent) change in their physical characteristics, are the means of choice in situations such as these.
Thermal paint is known, a paint which changes its color permanently as a function of temperature. Among the various disadvantages of this solution, the necessity should be stressed, in particular, for the apparatus to be disassembled before the parts to be investigated can be painted, the relative inaccuracy of the temperature determination, and the need for a closely monitored “extra run” of the apparatus, as a result of which the measurement does not reflect the normal operating conditions. One major disadvantage is that the chemical reaction which leads to the color change is not reversible. It is therefore possible to measure only the maximum temperature reached on each occasion throughout the entire operation.
Furthermore, metallographic temperature determination is known, but this is not non-destructive and is therefore not feasible for the stated applications.
In addition to the thermal paint technique, there are numerous other experiments involving the part of the apparatus to be investigated being provided with a thick-film coating which has thermal memory characteristics, that is to say which uses a stable change in the molecular structure with a number of the high-temperature operations to which it was most recently subjected.
An ideally reversible process (reversible in the sense that the process follows the operating temperature and does not just register the highest temperature measured in each case) leads to a measurable change in the physical characteristics (variables, parameters), which may be electrical (resistivity), magnetic (permeability, magnetization, magnetic saturation), mechanical (hardness, modulus of elasticity, Poissin ratio, yield point, strain, density), acoustic (compression wave velocity, shear-wave velocity, bar velocity), thermal (thermal conductivity and strain) or optical (color change). Once the coating has cooled down, the aim is to determine the last high-temperature operation by subjecting the parts investigated to various heat treatments, and by finding the sought temperature through the use of empirical interpolation rules.